Deployment

Before describing how to organize your source code directories,
it is useful to examine the runtime organization of a web application.
Prior to the Servlet API Specification, version 2.2, there was little
consistency between server platforms. However, servers that conform
to the 2.2 (or later) specification are required to accept a
Web Application Archive in a standard format, which is discussed
further below.

A web application is defined as a hierarchy of directories and files
in a standard layout. Such a hierarchy can be accessed in its "unpacked"
form, where each directory and file exists in the filesystem separately,
or in a "packed" form known as a Web ARchive, or WAR file. The former format
is more useful during development, while the latter is used when you
distribute your application to be installed.

The top-level directory of your web application hierarchy is also the
document root of your application. Here, you will place the HTML
files and JSP pages that comprise your application's user interface. When the
system administrator deploys your application into a particular server, he
or she assigns a context path to your application (a later section
of this manual describes deployment on Tomcat). Thus, if the
system administrator assigns your application to the context path
/catalog, then a request URI referring to
/catalog/index.html will retrieve the index.html
file from your document root.

To facilitate creation of a Web Application Archive file in the required
format, it is convenient to arrange the "executable" files of your web
application (that is, the files that Tomcat actually uses when executing
your app) in the same organization as required by the WAR format itself.
To do this, you will end up with the following contents in your
application's "document root" directory:

*.html, *.jsp, etc. - The HTML and JSP pages, along
with other files that must be visible to the client browser (such as
JavaScript, stylesheet files, and images) for your application.
In larger applications you may choose to divide these files into
a subdirectory hierarchy, but for smaller apps, it is generally
much simpler to maintain only a single directory for these files.

/WEB-INF/web.xml - The Web Application Deployment
Descriptor for your application. This is an XML file describing
the servlets and other components that make up your application,
along with any initialization parameters and container-managed
security constraints that you want the server to enforce for you.
This file is discussed in more detail in the following subsection.

/WEB-INF/classes/ - This directory contains any Java
class files (and associated resources) required for your application,
including both servlet and non-servlet classes, that are not combined
into JAR files. If your classes are organized into Java packages,
you must reflect this in the directory hierarchy under
/WEB-INF/classes/. For example, a Java class named
com.mycompany.mypackage.MyServlet
would need to be stored in a file named
/WEB-INF/classes/com/mycompany/mypackage/MyServlet.class.

When you install an application into Tomcat (or any other 2.2 or later
Servlet container), the classes in the WEB-INF/classes/
directory, as well as all classes in JAR files found in the
WEB-INF/lib/ directory, are made visible to other classes
within your particular web application. Thus, if
you include all of the required library classes in one of these places (be
sure to check licenses for redistribution rights for any third party libraries
you utilize), you will simplify the installation of your web application --
no adjustment to the system class path (or installation of global library
files in your server) will be necessary.

Much of this information was extracted from Chapter 9 of the Servlet
API Specification, version 2.3, which you should consult for more details.

Like most servlet containers, Tomcat also supports mechanisms to install
library JAR files (or unpacked classes) once, and make them visible to all
installed web applications (without having to be included inside the web
application itself). The details of how Tomcat locates and shares such
classes are described in the
Class Loader HOW-TO documentation.
The location commonly used within a Tomcat installation for shared code is
$CATALINA_HOME/lib. JAR files placed here are visible both to
web applications and internal Tomcat code. This is a good place to put JDBC
drivers that are required for both your application or internal Tomcat use
(such as for a JDBCRealm).

Out of the box, a standard Tomcat installation includes a variety
of pre-installed shared library files, including:

The Servlet 3.0 and JSP 2.2 APIs that are fundamental
to writing servlets and JavaServer Pages.

As mentioned above, the /WEB-INF/web.xml file contains the
Web Application Deployment Descriptor for your application. As the filename
extension implies, this file is an XML document, and defines everything about
your application that a server needs to know (except the context path,
which is assigned by the system administrator when the application is
deployed).

The complete syntax and semantics for the deployment descriptor is defined
in Chapter 13 of the Servlet API Specification, version 2.3. Over time, it
is expected that development tools will be provided that create and edit the
deployment descriptor for you. In the meantime, to provide a starting point,
a basic web.xml file
is provided. This file includes comments that describe the purpose of each
included element.

NOTE - The Servlet Specification includes a Document
Type Descriptor (DTD) for the web application deployment descriptor, and
Tomcat enforces the rules defined here when processing your application's
/WEB-INF/web.xml file. In particular, you must
enter your descriptor elements (such as <filter>,
<servlet>, and <servlet-mapping> in
the order defined by the DTD (see Section 13.3).

A /META-INF/context.xml file can be used to define Tomcat specific
configuration options, such as an access log, data sources, session manager
configuration and more. This XML file must contain one Context element, which
will be considered as if it was the child of the Host element corresponding
to the Host to which the web application is being deployed. The
Tomcat configuration documentation contains
information on the Context element.

The description below uses the variable name $CATALINA_BASE to refer the
base directory against which most relative paths are resolved. If you have
not configured Tomcat for multiple instances by setting a CATALINA_BASE
directory, then $CATALINA_BASE will be set to the value of $CATALINA_HOME,
the directory into which you have installed Tomcat.

In order to be executed, a web application must be deployed on
a servlet container. This is true even during development.
We will describe using Tomcat to provide the execution environment.
A web application can be deployed in Tomcat by one of the following
approaches:

Copy unpacked directory hierarchy into a subdirectory in directory
$CATALINA_BASE/webapps/. Tomcat will assign a
context path to your application based on the subdirectory name you
choose. We will use this technique in the build.xml
file that we construct, because it is the quickest and easiest approach
during development. Be sure to restart Tomcat after installing or
updating your application.

Copy the web application archive file into directory
$CATALINA_BASE/webapps/. When Tomcat is started, it will
automatically expand the web application archive file into its unpacked
form, and execute the application that way. This approach would typically
be used to install an additional application, provided by a third party
vendor or by your internal development staff, into an existing
Tomcat installation. NOTE - If you use this approach,
and wish to update your application later, you must both replace the
web application archive file AND delete the expanded
directory that Tomcat created, and then restart Tomcat, in order to reflect
your changes.

Use the Tomcat "Manager" web application to deploy and undeploy
web applications. Tomcat includes a web application, deployed
by default on context path /manager, that allows you to
deploy and undeploy applications on a running Tomcat server without
restarting it. See Manager App HOW-TO
for more information on using the Manager web application.

Use "Manager" Ant Tasks In Your Build Script. Tomcat
includes a set of custom task definitions for the Ant
build tool that allow you to automate the execution of commands to the
"Manager" web application. These tasks are used in the Tomcat deployer.

Use the Tomcat Deployer. Tomcat includes a packaged tool
bundling the Ant tasks, and can be used to automatically precompile JSPs
which are part of the web application before deployment to the server.

Deploying your app on other servlet containers will be specific to each
container, but all containers compatible with the Servlet API Specification
(version 2.2 or later) are required to accept a web application archive file.
Note that other containers are NOT required to accept an
unpacked directory structure (as Tomcat does), or to provide mechanisms for
shared library files, but these features are commonly available.